1. Field of the Invention
The present invention relates to a printed circuit board manufacturing method having in particular a characteristic in the through-hole formation, a printed circuit board manufactured by the manufacturing method, and an electronic apparatus equipped with such a printed circuit board.
2. Description of the Related Art
Fabrication of an electronic circuit by mounting electronic components on a printed circuit board is widely performed. The electronic circuit performs a required operation by means of wiring between circuits within the electronic components and signal lines on the printed circuit board.
On a printed circuit board, through holes are formed for the purpose of mounting electronic components or connecting wires formed on different layers of the printed circuit board, and a conductive film is formed on an inner wall of the through hole by a process such as plating to connect the wires in plural layers. Also, electronic components are connected to wires of the printed circuit board by soldering after leads of the electronic components are inserted into the through holes.
Here, in a case of connecting wires in plural layers and carrying a signal at high speed with a conductive film of the inner wall of the through holes, for the purpose of reducing the transfer loss caused by the conductive film of the through holes, the conductive film of the inner wall of the through holes is not formed over the entire span of the printed circuit board of the through holes from the surface to the underside, but is removed in unnecessary places and left behind on portions necessary for the connection of high-speed-transfer-use wires through a plurality of layers.
FIG. 1 shows the appearance of eliminating conductive film formed in the through holes of the printed circuit board. FIG. 2 shows the appearance of having eliminated a portion of the conductive film formed in the through holes of the printed circuit board. In the FIGS. 1 and 2, only one through hole 11 of a printed circuit board 10 is shown as an example.
Here, a conductive film 111 is formed on the inner wall of the through hole 11, and a wire 101 and a wire 102 in two levels, of among wires 101, 102, 103, 104 of 4 levels, are set into connection with the conductive film 111. At this time, first, the conductive film is once formed over the entire area, that is, the entire area from the surface of the printed circuit board 10 to the underside thereof, of the inner wall of the through hole 11, and after this, the conductive film 111 formed on the inner wall of the through hole 11 is removed with a drill 20 as shown in FIG. 1, thus leaving behind the conductive film 111 on the inner wall of the through hole 11 as shown in FIG. 2 only in a portion that connects the wires 101 and 102 in the two layers. In this case, there is formed a difference-in-level 112 between the portion of the inner wall of the through hole 11 treated by the drill, and the portion un-treated by the drill, even if the conductive film 111 is not considered. By doing this, it is possible, with the wires 101 and 102, to reduce transfer loss of the transferred high speed signal in the through holes 11.
However, in this method, the conductive layer is eliminated after it has been formed in the inner wall of the through holes. This increases the number of processes, thereby increasing costs, and there is also a concern that remnants resulted from insufficient elimination of the conductive film are very likely to cause a short-circuit between wires.
In Japanese Patent Application Publication No. 2003-204157, there is proposed a technique that, for the purpose of solving the above problem, buries a plating prevention layer in the printed circuit board to which a conductive film cannot adhere by plating or the like, and the conductive film is cut off at a portion of the inner wall of the through hole on which portion the plating prevention layer appears.
Also, in Japanese Patent Application Publication No. 1990-143588, there is proposed a technique that forms an intermediate single layer of an insulation plate in a bulky printed circuit board, the insulation plate being formed of a material that cannot be treated by plating, thereby preventing the conductive film from being formed at a portion of the inner wall of the through hole where this insulation plate appears.
However, in the technique proposed in the above mentioned Japanese Patent Application Publication No. 2003-204157, since the conductive film of the inner wall of the through hole is separated only by the end face of the thin plating prevention layer, electronic coupling occurs in cases of high speed signals and thus, it is very likely that transfer loss cannot be sufficiently reduced.
And in the technique proposed in the above mentioned Japanese Patent Application Publication No. 1990-143588, because the conductive film between the through holes is separated only by the insulation plate and thus, coupling is very likely to occur even in a case of a high speed signal, it is possible to reduce transfer loss. However, with this technique, since in any through hole, the conductive film on the inner wall is separated by the insulation plate, it is impossible to separate only the conductive film of the through hole connecting the wires handling high speed signal transfer. Therefore, this technique cannot be used in a case where through holes in which the conductive film of the inner wall should be separated and through holes in which the conductive film of the inner wall should not be separated are interspersed in a printed circuit board.